Boron nitride nanotubes (BNNTs) have been found to exhibit many interesting and practically useful properties, and BNNTs have therefore been attracting more and more attention. Such properties include:                excellent thermal conductivity;        excellent mechanical properties;        strong oxidation resistance and high temperature stability (chemically benign at high temperature);        stable wide bandgaps (close to 6 eV);        excellent radiation shielding (due to the presence of isotopic 10B);        excellent piezoelectric properties; and        optoelectronic properties.        
With such premium and desirable properties BNNTs have a wide range of potential applications. However, a typical requirement then is that it is necessary to provide the BNNTs in high-purity and large quantities, and as thin films at selected locations (this is critical for integration into functional devices) with appropriately high density and with appropriately high purity. Furthermore, it is also important that the BNNTs can be provided on a commercial production scale and economically. The present invention seeks to provide a process for providing BNNTs that meets these various requirements. In an embodiment of the invention the BNNTs are produced on a substrate.
Accordingly, in one embodiment the present invention provides a process for producing boron nitride nanotubes, which process comprises heating a liquid composition comprising boron particles and a metal compound, wherein heating takes place at a temperature of from 800-1300° C. in a gaseous atmosphere containing nitrogen that causes boron nitride nanotubes to grow, and wherein the boron particles have an average particle size of less than 100 nm, and wherein the metal compound is selected such that it promotes the growth of boron nitride nanotubes during heating. The liquid composition comprises a vehicle in which the boron particles are dispersed. The metal compound may be dispersed or preferably dissolved in the vehicle.
In this embodiment of the invention BNNTs can be grown as loose form (clusters) by heating of the ink in a suitable vessel/boat. The vehicle in the ink vaporises rapidly during heating (if ethanol is used this vapourises at about 90° C.). Solid B and metal nanoparticles precipitate, and they react with nitrogen at 1100° C. The BNNTs may then be harvested and used as necessary.
In accordance with the present invention it is possible to grow BNNTs on a substrate by application of boron particles and metal compound to the substrate, at least the boron particles being provided as a liquid formulation. The present invention will be discussed with particular emphasis on this embodiment although it will be appreciated that much of the following discussion will have applicability to the embodiment in which BNNTs are grown from within the bulk of a liquid composition rather than on the surface of a substrate.
Accordingly, in one embodiment the present invention provides a process for producing boron nitride nanotubes on a substrate, which process comprises the following steps:    (a) applying to the substrate a metal compound and boron particles, the boron particles having an average particle size of less than 100 nm and wherein the boron particles are applied in the form of a liquid composition comprising the boron particles dispersed in a vehicle;    (b) heating the substrate at a temperature of from 800-1300° C. in a gaseous atmosphere containing nitrogen thereby causing boron nitride nanotubes to grow on the substrate; and            wherein the metal compound is selected such that it promotes the growth of boron nitride nanotubes during heating step (b).        
In another embodiment, the present invention provides a process for producing boron nitride nanotubes on a substrate which process comprises the following steps:    (a) coating the substrate with a metal compound dissolved or dispersed in a vehicle;    (b) applying to the coated substrate an ink comprising boron particles having an average particle size of less than 100 nm; Using injet printing, brush painting, spraying and other processes    (c) heating the substrate at a temperature of from 800-1300° C. in a gaseous atmosphere containing nitrogen thereby causing boron nitride nanotubes to grow on the substrate, in rorating/tilting tube furnaces            wherein the metal compound is selected such that it promotes the growth of the boron nitride nanotubes during heating in step (c).        
In accordance with the present invention it has been found that the use of particular metal compounds is effective in promoting the growth of BNNTs from boron particles provided on a substrate. Without wishing to be bound by theory it is believed that during the heating step the metal compound is reduced to the corresponding metal and it is this metal that acts as a catalyst (or nucleation site) that promotes the formation and growth of BNNTs from the boron particles provided on the substrate surface.
Herein the term “liquid composition” is used interchangeably with the term “ink”.
The use of an ink containing boron particles is also an important aspect of the present invention. Use of the ink enables (at least) the boron particles to be applied onto a substrate by a variety of techniques (such as inkjet printing, brushing/painting, spraying and the like) and also allows coating according to any desired configuration. It has also been found that certain vehicles used for formulation of the ink can have a beneficial effect on the production and growth of the BNNTs.
The present invention also provides an ink comprising boron particles that is suitable for use in the process of the invention.
The present invention also provides a substrate comprising on a surface of the substrate boron nitride nanotubes produced in accordance with the present invention. The boron nitride nanotubes may take the form of a film on the substrate. The invention also extends to the use of such coated substrates.